Save the game as frequently as possible. You will avoid frustrating necessity to replay the same location for the second time after the death of your party. Important - Don't rely solely on automatic and quick saves. You should also create standard savegames regularly, especially after arriving in a brand new location. Doing this will allow you to avoid problems caused by wrong choices or by encountering a bug in the game that may interfere with the progress.
Think before picking a difficulty level. If you don't have experience with other games from same genre, start the game on easy difficulty. You can change your decision at any moment of the game. Two additional modes set before a new game are exception: Expert mode (it disables all hints and other help functions) and Trial of Iron - they cannot be changed later in the game. The Trial of Iron mode forces the player to play the game using only a single saved state (creating saved games on your own is not possible). This means that if all of your team members die the game ends and you're forced to repeat it from the very beginning. Obviously you shouldn't be choosing this difficulty level during your first playthrough. Consider playing in this mode once you've perfected the game and when you know what to expect in all the main locations.
Read dialogues and descriptions of the world. Not only will you know where to go and what to do, but you will become more immersed in the game and it will become more fun to play.
Explore the game world. Gather all herbals and resources (use the TAB key to highlight all interactive objects from the surrounding area). Your inventory has unlimited space and gathered items can be later used for weapon upgrading and creating potions. You can sell then as well.
Rest in inns whenever you need. For a rather small price you will receive stats bonus without the need of using camp materials. Resting in your stronghold is free and after upgrading it gives various temporary stats boost (although lover than in inns).
Rely on camping supplies when going on longer trips. They're your best help if retreating to the keep or to the nearest town would be time-consuming or when you're know that you're about to take part in a difficult battle with a boss or a group of strong enemies. Sadly you're allowed to store a very limited number of camping supplies - no more than 6 for the easy difficulty setting, no more than 4 for normal and only 2 for the high difficulty level. You can replenish your camping supplies by finding them in the game world or by buying them from the merchants. Important - Your team is not allowed to rest using camping supplies if it's engaged in battle or if it has been spotted by one of the enemies. Additionally you may be forbidden from using camping supplies if you're in a city or inside a small building.
While being in buildings and dungeons, search chests and barrels. Many time you will find something interesting inside.
When exploring, use scouting mode. If you see something suspicious or you're in narrow locations (dungeons, rooms), use scouting mode that not only allows you to see traps, but hidden lockers and switches as well. For example, it allows you to find a hidden door that will lead you to a shortcut or inaccessible in other way treasure.
Sometimes valuable items can be found in not so obvious places like bookshelves or in vases.
Complete side quests. Not only they increase your party level, but give you dozens of hours of additional gameplay.
Talk with characters that have unique name. If a character name is marked with yellow color, it means you can "touch" his soul and read a short history of that person. Characters that give side quests usually talk when you walk near them.
Carry with you lockpick, rope with grappling hook, hammer and chisel. You should have two or three pieces of each. It will allow you to get to hardly accessible places. You can purchase all of the tools mentioned above from certain merchants, however you will also find plenty of them as a result of careful exploration.
Visit your stronghold from time to time. Try to expand it regularly. You will increase possibility of event occurrence and will gain other profits.
Use crafting - alchemy, enchanting and scroll making. You know all the recipes from the beginning of the game, all you need is to get required level. By using potions or scrolls you improve your chances when fighting stronger enemies. Certain requirements must be met to create some items, like using spells or having specific lore level.
Learn how to manipulate time. Not the active pause during combat, but the possibility to speed and slow the game speed. You can speed up the game when you travel through already explored location and you want to get to a certain point, by doing it you will avoid waiting. Slowing the game is useful when sneaking - it gives you certainty that you will see enemy before he sees you. By default you control time with the "s" and "d" keys.